1. Field of the Invention
The present invention relates to an exhaust gas purification system including a catalyst that is provided in an exhaust gas passage of an internal combustion engine and that can be regenerated by redispersing platinum particles having been sintered.
2. Description of the Related Art
In automobiles, an exhaust gas purifying catalyst is used to remove pollutants, such as HC, CO and NOx, contained in an exhaust gas from an engine. A known example of such an exhaust gas purifying catalyst is a three-way catalyst that simultaneously removes HC, CO and NOx from the exhaust gas after combustion at a theoretical air-fuel ratio. The three-way catalyst includes a honeycombed carrier base made of, for example, cordierite or a metal foil, a catalyst-carrying layer made of, for example, active alumina powders or silica powders and coated on the surface of the carrier base, and a precious metal such as platinum carried on the catalyst-carrying layer. The three-way catalyst removes HC and CO in the exhaust gas by oxidation, and reduces NOx.
An oxidation catalyst is also known, in which a precious metal is carried on a catalyst-carrying layer made of zeolite, which has excellent in absorption characteristics for HC contained in the exhaust gas. In the oxidation catalyst, HC contained in the exhaust gas is absorbed by the catalyst-carrying layer at low temperatures, and is released as the temperature of the catalyst increases. The released HC is oxidized by the precious metal when the precious metal is at or above its active temperature. In this way, HC emission can be suppressed at low temperatures, for example, when starting the engine and during in winter.
In addition, to mitigate variations in atmosphere of the exhaust gas, a catalyst having oxygen storage/releasing capacity and an NOx storage reduction catalyst are known. In the former, an element such as ceria is carried on a catalyst-carrying layer. In the latter, an NOx storage material such as alkali metals and alkali earth metals is carried on a catalyst-carrying layer, along with a precious metal.
For the precious metal for use as an active component of such catalysts, platinum (Pt) is primarily used. When platinum is exposed to an oxidizing atmosphere at a high temperature for a long time, however, platinum particles become flocculated (platinum particles grow) and thus platinum sinters, unfavorably reducing the specific surface area of the platinum particles and hence the activity of the catalyst. Thus, various methods have been developed to regenerate exhaust gas purifying catalysts in which the platinum has been sintered.
For example, Japanese patent application publication No. JP-A-2000-202309 describes a method for regenerating an exhaust gas purifying catalyst. The catalyst is made up of a carrier containing at least one element selected from alkaline earth metal oxides and rare earth element oxides, and platinum carried on the carrier. The method includes an oxidation process in which the catalyst is heated in an oxidizing atmosphere containing oxygen, and a reduction process conducted after the oxidation process. In this regeneration method, platinum oxide is formed on the surface of platinum metal particles having been sintered and grown, in the oxidation process. The platinum oxide interacts strongly with the carrier containing the specific oxide mentioned above. Thus, the platinum oxide moves from the surface of the platinum metal particles to the surface of the carrier, allowing metal platinum to appear on the surface of the platinum metal particles. The metal platinum is oxidized into platinum oxide, which in turn moves to the surface of the carrier. By repeating these events, the platinum metal particles being carried become gradually smaller in diameter and dispersed over the surface of the carrier, resulting in platinum oxide being highly dispersed over the carrier. The catalyst is then subjected to the reduction process, in which the platinum oxide is reduced to metal platinum. The activity of the catalyst can be restored by the highly dispersed metal platinum.
The above-described regeneration method includes, as the oxidation process to regenerate platinum, increasing the air-fuel (A/F) ratio of an air-fuel mixture flowing into an engine to an extremely high degree. When the above-described regeneration method is used to regenerate a catalyst incorporated in an exhaust gas purification system, however, the time when regeneration of platinum is required does not always coincide with the time when the air-fuel ratio is increased, depending on the operating conditions, making it difficult to reliably regenerate platinum.
When the concentration of oxygen is low, a sufficient amount of oxygen may not reach the downstream side of the catalyst, even if the air-fuel ratio is increased. In such a case, the regeneration process cannot be performed efficiently. Providing an air supply device on the upstream side of the catalyst is also proposed. With such an arrangement, however, it is difficult for an ordinary three-way catalyst to remove NOx in the exhaust gas with an excessive amount of oxygen supplied. As a result, NOx may unfavorably be emitted.